Flat panel display

ABSTRACT

A high-speed flat panel display having a long lifetime. Thin film transistors in a pixel portion having a plurality of pixels are contacted differently from thin film transistors in driving circuit portions for driving the pixels, thereby enhancing luminance uniformity and reducing power consumption. The thin film transistors each have a channel region and a body contact region for applying a predetermined voltage to the channel region. At least one thin film transistor in the pixel portion is a source-body contact thin film transistor having the body contact region connected to one of source and drain electrodes so that the predetermined voltage can be provided to the channel region. Each thin film transistor in the driving circuit portion is a gate-body contact thin film transistor having the body contact region connected to the gate electrode so that a predetermined voltage can be provided to the channel region.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/938,000, filed Sep. 10, 2004 now U.S. Pat. No. 7,450,100 which claimspriority to and the benefit of Korean Patent Application No. 2003-64895,filed Sep. 18, 2003 in the Korean Intellectual Property Office, thedisclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flat panel display and, moreparticularly, to a high-speed flat panel display having a long lifetimein which thin film transistors of a pixel portion and driving circuitportions have different body contact structures.

2. Description of the Related Art

Generally, an active matrix organic light emitting diode (AMOLED)display includes a pixel portion in which an array of thin filmtransistors is arranged, and a data driving circuit portion and a gatedriving circuit portion for driving the array of thin film transistorsin the pixel portion.

In a conventional AMOLED display, the thin film transistors in the pixelportion and thin film transistors in the data or gate driving circuitportion are all composed of polysilicon thin film transistors.Therefore, in a 180 ppi or higher resolution AMOLED display, if thepixel portion and the driving circuit portions are composed of thepolysilicon thin film transistors, a high-speed operating characteristicof the driving circuit portions could be obtained because of highmobility of the thin film transistors. However, because an on-current isvery high, the amount of a current flowing through EL elements in thepixel portion exceeds a limit value, and thus luminance per unit areaincreases, which shortens the lifetime of the EL elements.

Meanwhile, when the pixel portion and the driving portions are composedof thin film transistors with low mobility to maintain a suitableon-current characteristic, it results in a relatively low on-current andthus causes appropriate luminance, which overcomes the shortenedlifetime problem of the EL element, but it cannot support a high-speedoperating characteristic of the driving circuit portions.

SUMMARY OF THE INVENTION

Accordingly, in exemplary embodiments of the present invention, ahigh-speed flat panel display having a long lifetime is provided. Theflat panel display has a pixel portion and driving circuit portions thatare composed of thin film transistors having different body contacts,thereby enhancing luminance uniformity and reducing power consumption.Such a flat panel display may be suitable for small size displays havinghigh resolution.

In an exemplary embodiment of the present invention, a flat paneldisplay including a pixel portion in which a plurality of pixels arearranged; and driving circuit portions for driving the pixels, isprovided. The pixel portion and the driving circuit portions eachinclude thin film transistors each having a channel region and a bodycontact region for applying a predetermined voltage to the channelregion. The body contact region of at least one of the thin filmtransistors in the pixel portion is contacted differently from the bodycontact region of the thin film transistors in the driving circuitportions.

The at least one of the thin film transistors in the pixel portion maybe a source-body contact thin film transistor and may further include agate electrode, a source electrode, a drain electrode and source anddrain regions connected, respectively, to the source and drainelectrodes. The body contact region of the at least one of the thin filmtransistors may be connected to one of the source and drain electrodesso that the predetermined voltage from the connected electrode can beprovided to the channel region. The body contact region may include animpurity region which is doped with an impurity of an oppositeconductivity type from impurities in the source and drain regions.

Each of the thin film transistors in the driving circuit portions may bea gate-body contact thin film transistor, and may further include a gateelectrode, a source electrode, a drain electrode, and source and drainregions connected, respectively, to the source and drain electrodes. Thebody contact region of each of the thin film transistors in the drivingcircuit portions may be connected to the gate electrode so that thepredetermined voltage from the gate electrode can be provided to thechannel region. The body contact region may include an impurity regionwhich is doped with an impurity of opposite conductivity type fromimpurities in the source and drain regions.

The thin film transistors in the pixel portion may include a first thinfilm transistor which is switched by a gate driving signal to deliver adata signal, and a second thin film transistor for driving an EL elementaccording to the data signal delivered via the first thin filmtransistor. The body contact region of at least one of the first thinfilm transistor and the second thin film transistor may be contacteddifferently from the body contact region of the thin film transistors inthe driving circuit portions.

The thin film transistors in the pixel portion may be NMOS or PMOS thinfilm transistors and the thin film transistors in the driving circuitportions may be PMOS or NMOS thin film transistors. Alternatively, eachof the thin film transistors in the pixel portion may be either an NMOSthin film transistor or a PMOS thin film transistor, and each of thethin film transistors in the driving circuit portions may be a PMOS thinfilm transistor or an NMOS thin film transistor based on a CMOStechnology.

In another exemplary embodiment of the present invention, there isprovided a flat panel display including a pixel portion in which aplurality of pixels are arranged, and driving circuit portions fordriving the pixels. The pixel portion includes thin film transistorseach having a substantially uniform output current over a predeterminedrange of input voltages, and the driving circuit portions include thinfilm transistors each having a suitable ON/OFF characteristic at a lowinput voltage.

Each of the thin film transistors in the pixel portion may be asource-body contact thin film transistor and may include an active layerhaving a channel region and a body contact region for providing apredetermined voltage to the channel region, a gate electrode, a sourceelectrode and a drain electrode. The body contact region may beconnected to one of the source electrode and the drain electrode so thatthe predetermined voltage from the connected electrode can be providedto the channel region. A drain current outputted via the drain electrodemay be substantially uniform with respect to an input voltage applied tothe drain electrode.

Each of the thin film transistors in the driving circuit portions may bea gate-body contact thin film transistor, and may include an activelayer having a channel region and a body contact region for providing apredetermined voltage to the channel region, a gate electrode, a sourceelectrode and a drain electrode. The body contact region may beconnected to the gate electrode so that the predetermined voltage fromthe gate electrode can be provided to the channel region. A draincurrent outputted via the drain electrode may have a suitable ON/OFFcharacteristic with respect to an input voltage applied to the gateelectrode.

In yet another exemplary embodiment of the present invention, a flatpanel display includes a pixel portion including a plurality of pixelcircuits, each comprising at least one first thin film transistor havinga first body contact region, a first channel region connected to thefirst body contact region, a source electrode and a drain electrode,wherein the first body contact region is connected to the sourceelectrode or the drain electrode, such that a first predeterminedvoltage applied at the connected electrode is applied to the firstchannel region through the first body contact region; and a drivingcircuit portion for driving the pixel circuits, the driving circuitportion including a plurality of second thin film transistors, eachhaving a second body contact region, a second channel region connectedto the second body contact region, and a gate electrode, wherein thesecond body contact region is connected to the gate electrode, such thata second predetermined voltage applied at the gate electrode is appliedto the second channel region through the second body contact region.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features in exemplary embodiments of the presentinvention will be described in reference to certain exemplaryembodiments thereof with reference to the attached drawings in which:

FIG. 1 is a block diagram of an organic light emitting diode (OLED)display device according to an exemplary embodiment of the presentinvention;

FIG. 2 is a plan view of a thin film transistor in a pixel portion in anorganic light emitting diode according to an exemplary embodiment of thepresent invention;

FIG. 3A is a cross-sectional view of the thin film transistor in thepixel portion taken along line 2A-2A′ of FIG. 2;

FIG. 3B is a cross-sectional view of the thin film transistor in thepixel portion taken along line 2B-2B′ of FIG. 2;

FIGS. 4A and 4B are graphs illustrating a relationship between a drainvoltage and a drain current in the thin film transistor of the pixelportion shown in FIG. 2;

FIG. 5 is a plan view of a thin film transistor of a driving circuitportion in an organic light emitting diode according to an embodiment ofthe present invention; and

FIGS. 6A and 6B are graphs illustrating a relationship between a gatevoltage and a drain current in the thin film transistor of the drivingcircuit portion shown in FIG. 5.

DETAILED DESCRIPTION

Referring to FIG. 1, an OLED display device includes a pixel portion 2in which a plurality of pixels are arranged in a matrix form, anddriving circuit portions for driving the pixel portion 2 on aninsulating substrate 1. Although the pixel portion 2 is not shown indetail in FIG. 1, a plurality of gate lines, a plurality of data lines,a plurality of power lines, and a plurality of pixels connected to thelines are arranged in a matrix form. Each pixel is basically composed ofan electroluminescent (EL) element, a driving transistor for supplying adriving current according to a data signal from a data line to the ELelement, a switching transistor for delivering the data signal to thedriving transistor in response to a scan signal applied to a gate line,a capacitor for storing the data signal, and the like.

The driving circuit portions for driving the pixels in the pixel portion2 include a gate driving circuit portion 4 for providing a scan signalfor driving the gate lines in the pixel portion 2, and a data drivingcircuit portion 3 for supplying the data signals to data lines in thepixel portion 2.

In an exemplary embodiment of the present invention, each of the thinfilm transistors in the pixel portion 2 is a source-body contact thinfilm transistor, and each of the thin film transistors of the drivingcircuit portions 3 and 4 is a gate-body contact thin film transistor.The source-body contact thin film transistor has a body contactstructure including a body contact region separately formed from thesource and drain regions on an active layer. The body contact region isconnected to either the source region or the drain region, for example,the source region. The gate-body contact thin film transistor has a bodycontact structure including a body contact region separately formed fromthe source and drain regions on the active layer. The body contactregion is connected to the gate electrode.

Since identical drain currents can be obtained over a wide range ofdrain voltages in the source-body contact thin film transistor byreducing a kink effect, the source-body contact thin film transistor issuitable for thin film transistors making up the pixel portion 2. On theother hand, since the gate-body contact thin film transistor canimplement an ON/OFF characteristic at a low gate voltage, it is suitablefor thin film transistors making up the driving circuit portions 3 and4.

An exemplary source-body contact thin film transistor that can be usedas the thin film transistors in the pixel portion 2 is described inKorean Patent Application No. 2003-0027339, the entire content of whichis incorporated by reference herein. Further, an exemplary gate-bodycontact thin film transistor that can be used as the thin filmtransistors of the driving circuit portions 3 and 4 is described inKorean Patent Application No. 2003-0056594.

FIG. 2 is a plan view of a source-body contact thin film transistormaking up the pixel portion in the OLED display device according to anexemplary embodiment of the present invention as shown in FIG. 1, andFIGS. 3A and 3B show cross-sectional structures taken along lines 2A-2A′and 2B-2B′ of FIG. 2, respectively.

Referring to FIG. 2 and FIGS. 3A and 3B, a source-body contact thin filmtransistor used in an exemplary embodiment of the present inventionincludes an active layer 30, a gate electrode 50, and source and drainelectrodes 71 and 73. The source-body contact thin film transistor isformed on an insulating substrate 10, and an insulation layer 60separates the gate electrode 50 from the source and drain electrodes 71and 73. The active layer 30 includes source and drain regions 31 and 33with a channel region 35 formed therebetween, and a body contact region37 separately formed from the source and drain regions 31 and 33.

The gate electrode 50 is formed corresponding to the channel region 35of the active layer 30. The source electrode 71 is formed correspondingto the source region 31 and is electrically connected to the impurityregion for the source 31 via a contact 61. The drain electrode 73 isformed corresponding to the drain region 33, and is electricallyconnected to the impurity region for the drain 33 via a contact 63.Meanwhile, a connection wiring 77 is formed corresponding to the bodycontact region 37, and it electrically connects the body contact region37 to the source electrode 71 via a contact 67.

Further, although the connection wiring 77 for applying a power to thebody contact region 37 is formed integrally with the source electrode 71in the described exemplary embodiment, it may be separated from thesource electrode 71, and the same power as that applied to the sourceelectrode 71 may be applied to it. Further, although the connectionwiring 77 is formed to connect to the source electrode 71, it may beformed instead to connect to the drain electrode 73.

In the source-body contact thin film transistor having the above-statedstructure according to the exemplary embodiment of the presentinvention, hot carriers generated during normal operation at aninterface between the drain region 33 and the channel region 35 by alateral electric field in the drain region, are forced to go out throughthe body contact region 37. As a result, the hot carriers are preventedfrom moving into the source region 31, and thus a kink effect issuppressed.

FIGS. 4A and 4B are graphs that show operating characteristics of aconventional floating body thin film transistor (TFT) and a TFT having abody contact region used in the described exemplary embodiment. FIG. 4Ashows an I_(D)-V_(D) characteristic of the present invention and theprior art in an n type thin film transistor in which W/L=4 μm/4 μm andthe width of a Lightly Doped Drain (LDD) region is 1 μm. FIG. 4B showsan I_(D)-V_(D) characteristic of the described exemplary embodiment andthe prior art in a p type thin film transistor in which W/L=4 μm/4 μm.

Referring to FIGS. 4A and 4B, it can be seen that the source-bodycontact thin film transistor used in the described exemplary embodimenthas a better kink free characteristic as compared to a conventional TFTin which the active layer thereof is floated. At this time, a differencein the I_(D)-V_(D) characteristic between the n type TFT and the p typeTFT results because an impact ionization characteristic of holes is lessthan that of electrons.

FIG. 5 is a plan view of a gate-body contact thin film transistor usedin a driving circuit portion in an OLED display device according to anexemplary embodiment of the present invention.

Referring to FIG. 5, the gate-body contact thin film transistor used inan exemplary embodiment of the present invention includes an activelayer 130, a gate electrode 150, and source and drain electrodes 171 and173. The active layer 130 includes source and drain regions 131 and 133with a channel region 135 formed therebetween, and a body contact region137 separately formed from the source and drain regions 131 and 133.

The gate electrode 150 is formed corresponding to the channel region 135of the active layer 130. The source electrode 171 is formedcorresponding to the source region 131 and is electrically connected tothe impurity region for the source 131 via a contact 161. The drainelectrode 173 is formed corresponding to the drain region 133, and iselectrically connected to the impurity region for the drain 133 via acontact 163.

Further, a connection wiring 180 is formed corresponding to the bodycontact region 137. The connection wiring 180 couples the body contactregion 137 to the gate electrode 150 through a contact 165 formed on thegate electrode 150 and a contact 167 formed on the body contact region137. The connection wiring 180 is formed of the same material as that ofthe source or drain electrode 171 or 173, and has an island shapeconnection pattern.

Further, in the exemplary embodiment of the present invention, theconnection wiring 180 for applying power to the body contact region 137is electrically connected to the gate electrode 150 via the contact 165,which enables a low voltage drive, thereby reducing the swing width of athreshold voltage and deriving a high drain current at a low gatevoltage.

FIGS. 6A and 6B are graphs that show an operating characteristic of aconventional floating body thin film transistor and a gate-body contactthin film transistor used in the present invention. FIG. 6A illustratesa drain current I_(D) with respect to a gate voltage V_(G) in the casewhere each of the floating body thin film transistor and the gate-bodycontact thin film transistor is an NMOS transistor, and FIG. 6Billustrates a drain current I_(D) with respect to a gate voltage V_(G)in the case where each of the floating body thin film transistor and thegate-body contact thin film transistor is a PMOS transistor.

Referring to FIGS. 6A and 6B, since a threshold voltage of the gate-bodycontact thin film transistor has a steeper slope than that of thefloating body thin film transistor, an ON/OFF characteristic can beobtained at a low gate voltage.

Accordingly, in the exemplary embodiment of the present invention, thedriving circuit portions 3 and 4 are composed of the gate-body contactthin film transistor shown in FIG. 4, and the pixel portion 2 iscomposed of the source-body contact thin film transistor as shown inFIG. 2 and FIGS. 3 a and 3 b, thereby obtaining a high-speed operatingcharacteristic, while allowing a low voltage drive at the same time.Further, since a substantially uniform current flows through the ELelement in the pixel portion, it is possible to obtain a substantiallyuniform luminance characteristic and to expand the lifetime thereof.

In the exemplary embodiment of the present invention, the source anddrain regions and the body contact region are of different conductivetypes. For example, if the source and drain regions are composed of ahigh concentration n-type impurity region, the body contact region iscomposed of a high concentration p-type impurity region. On the otherhand, if the source and drain regions are composed of the highconcentration p-type impurity region, the body contact region iscomposed of the high concentration n-type impurity region. In thedescribed exemplary embodiment, the channel region in the active layeris an intrinsic region in which first or second conductive typeimpurities are not doped.

Further, although forming the body contact region in the thin filmtransistor in which the source and drain regions are composed of highconcentration impurity regions has been described in reference to theexemplary embodiments of the present invention, the principles of thepresent invention are applicable to a thin film transistor in whichsource and drain regions of the transistor have an LDD structure of ahigh concentration impurity region and a low concentration impurityregion.

In addition, the source-body contact thin film transistor and thegate-body contact thin film transistor in exemplary embodiment of thepresent invention are not limited to the illustrated structures and mayhave either or both a structure in which the source and the body contactregion are interconnected and a structure in which the gate and the bodycontact region are interconnected.

In the exemplary embodiment of the present invention, it is possible touse the source-body contact thin film transistor as both a switchingtransistor and a driving transistor of the pixel portion. Alternatively,it is possible to use the source-body contact as either a switchingtransistor or a driving thin film transistor, but not both. By way ofexample, the driving transistor may be a source-body contact thin filmtransistor, while the switching transistor is a conventional floatingbody thin film transistor.

According to the above-described embodiments of the present invention,the thin film transistor in the pixel portion is a source-body contactthin film transistor having an excellent drain current characteristic,and the thin film transistor in the driving circuit portion is agate-body contact thin film transistor having an excellent ON/OFFcharacteristic at a low voltage, thereby maintaining a substantiallyuniform current flowing through an EL element to obtain a substantiallyuniform luminance characteristic as well as obtaining a high-speedoperating characteristic, and extending the lifetime of the EL element.

Although the present invention has been described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various modifications and variations can be madeto the described exemplary embodiments without departing from the spiritor scope of the present invention defined in the following claims andequivalents thereof.

What is claimed is:
 1. A flat panel display comprising: a pixel portionin which a plurality of pixels are arranged; and driving circuitportions for driving the pixels, wherein the pixel portion comprisesthin film transistors each having a more uniform output current over anoperational range of input voltages than that of a floating-body thinfilm transistor having substantially similar characteristics, and thedriving circuit portions comprise thin film transistors each havingdrain current-gate voltage characteristics such that a threshold voltagehas a steeper slope than that of a floating-body thin film transistorhaving substantially similar characteristics.
 2. The flat panel displayaccording to claim 1, wherein each of the thin film transistors in thepixel portion is a source-body contact thin film transistor andcomprises an active layer having a channel region and a body contactregion for providing a predetermined voltage to the channel region, agate electrode, a source electrode, and a drain electrode, and whereinthe body contact region is connected to one of the source electrode andthe drain electrode so that the predetermined voltage from the connectedelectrode can be provided to the channel region.
 3. The flat paneldisplay according to claim 2, wherein in each of the thin filmtransistors in the pixel portion, a drain current outputted via thedrain electrode thereof is substantially uniform with respect to aninput voltage applied to the drain electrode.
 4. The flat panel displayaccording to claim 1, wherein each of the thin film transistors in thedriving circuit portions is a gate-body contact thin film transistorwhich comprises an active layer having a channel region and a bodycontact region for providing a predetermined voltage to the channelregion, a gate electrode, a source electrode, and a drain electrode, andwherein the body contact region is connected to the gate electrode sothat the predetermined voltage from the gate electrode can be providedto the channel region.
 5. The flat panel display according to claim 4,wherein in each of the thin film transistors in the driving circuitportions, a drain current outputted via the drain electrode thereof hasa suitable ON/OFF characteristic with respect to an input voltageapplied to the gate electrode.
 6. A flat panel display comprising: apixel portion in which a plurality of pixels are arranged; and drivingcircuit portions for driving the pixels, wherein the pixel portioncomprises thin film transistors each having a more uniform outputcurrent over an operational range of input voltages than that of afloating-body thin film transistor having substantially similarcharacteristics, and wherein the driving circuit portions comprise thinfilm transistors each comprising a channel region; and a body contactregion electrically connected to a gate electrode for applying a voltageto the channel region.